archive/tar: isolate regular and sparse file handling as methods
Factor out the regular file handling logic into handleRegularFile
from nextHeader. We will need to reuse this logic when fixing #15573
in a future CL.
Factor out the sparse file handling logic into handleSparseFile.
Currently this logic is split between nextHeader (for GNU sparse
files) and Next (for PAX sparse files). Instead, we move this
related code into a single method.
There is no overall logic change. Thus, no unit tests.